Acetic anhydride is a well-known raw material widely used in the chemical industry, which is mainly used for producing chemicals such as cellulose acetate and is an important raw material for synthesizing medicines, flavors, dyes, etc. There are currently three industrial processes for producing acetic anhydride, including the ketene process, the acetaldehyde oxidation process and the methyl acetate carbonylation process. Among these processes, the ketene process, which belongs to an old-fashioned process and is small in scale, is adopted by many manufacturers and is thus predominant; however, the largest-scale single process for commercially producing acetic anhydride at present is the methyl acetate carbonylation process due to the high energy consuming and other drawbacks of the ketene process.
The ketene process is carried out by dissociating one water molecule or methane from the raw material, acetic acid or acetone, at a high temperature to form ketene, which then reacts with acetic acid to form acetic anhydride. The reaction temperature of this process is up to 750° C.; therefore, this process will gradually go out of use in the future for its high energy-consuming demand.
The acetaldehyde oxidation process is carried out by using metals such as manganese, cobalt, nickel, copper, etc. as the catalyst and oxidizing acetaldehyde into peracetic acid, which further reacts with acetaldehyde to form acetic anhydride and the by-product, water. Acetic anhydride will further be hydrolyzed into acetic acid so that the yield of acetic anhydride will be reduced. Therefore, the product of the acetaldehyde oxidation process is the mixture of acetic anhydride and acetic acid.
The methyl acetate carbonylation process for producing acetic anhydride is an expanded application of the methanol carbonylation process for producing acetic acid. The methyl acetate carbonylation process for producing acetic anhydride is carried out by reacting methyl acetate with carbon monoxide to produce acetic anhydride in the presence of the transition metal catalyst (such as rhodium, nickel, cobalt, iridium, etc.) and the iodide promoter. The difference between the methyl acetate carbonylation process and the methanol carbonylation process is the water content of the reaction solution; the reaction solution of the former has to be kept in anhydrous conditions, while the reaction solution of the latter can have any water ratio of 1˜20 wt. %. Water has a great influence on the stability of the catalyst, and the high water content is advantageous to the stability of the catalyst. Therefore, the stability of the catalyst in the anhydrous system of the methyl acetate carbonylation process is a primary problem that should be overcome. In order to solve the problem, a promoter or a co-catalyst such as alkali metal, phosphonium salt, ammonium salt and transition metal catalysts can be added to promote the stability and activity of the catalytic system. In addition, the carbon monoxide feed gas in the methyl acetate carbonylation process for producing acetic anhydride must contain a small amount of hydrogen so as to maintain the activity of the rhodium catalyst.
U.S. Pat. No. 4,002,678 discloses a preparation of acetic anhydride under anhydrous conditions by using nickel and chromium as the catalyst and carbon monoxide and methyl acetate or dimethyl ether as the raw materials to carry out a carbonylation reaction in the presence of a halide and a trivalent organo-nitrogen compound or a trivalent organo-phosphorus compound. The reaction temperature is about 150° C. and the pressure is controlled within 1000 psi. The organo-nitrogen compound promoter includes 2-hydroxypyridine, 2-quinolinol, 8-quinolinol, 2,6-diaminopyridine, etc. However, according to the disclosure of this patent, the reaction requires a time of several hours to several tens hours, depending on the conditions, and the conversion rate is substantially low.
U.S. Pat. No. 4,115,444 discloses a process for preparing acetic anhydride, in which a Group VIII noble metal is used as the catalyst, together with multiple promoters comprising at least one metal of Groups IVB, VB, and VIB or a non-noble metal of Group VIII or their compounds and a trivalent organo-nitrogen compound or a trivalent organo-phosphorus compound. The catalyst can be rhodium or iridium, the metal promoter can be iron, cobalt, nickel, chromium, etc., and the organo-nitrogen compound promoter includes an amine, an imidazole, an imide, an amide, an oxime, etc., of which triethylamine, methyl imidazole, 2,6-dimethylpyridine, etc. are given in the examples. This patent discloses the performance of multiple promoters of iron, cobalt, nickel and chromium; however, the influence of alkali metal iodine salts and organic promoters on the reaction rate is never disclosed.
U.S. Pat. No. 4,430,273 discloses a process for making acetic anhydride, wherein methyl acetate or dimethylether as the raw material is reacted with carbon monoxide under anhydrous conditions, at temperatures of 77˜302° C. and under pressures of 1˜300 bar in the presence of a Group VIII noble metal as the catalyst, while at least one heterocyclic aromatic compound (a quaternary nitrogen atom) is added as the promoter. The added heterocyclic aromatic compounds as given in the examples are aromatic iodine salts and are mostly with simple structures such as N-methylpyridine, N-methylimidazole, etc. However, this patent describes neither what structures of heterocyclic aromatic compounds are effective and how much performance they improve, nor the influence of addition of metal iodide salts.
U.S. Pat. No. 4,536,354 discloses a process for preparing carboxylic acid anhydrides, in which nickel is used as the catalyst and a compound having the following structure is used as the promoter:

In the above structural formula, X represents phosphorus, arsenic or antimony, Y represents oxygen, sulphur or selenium, a and b are 0 or 1, R5 represents hydrogen or a substituted or non-substituted hydrocarbon group, and R6 and R7 represent a substituted or non-substituted hydrocarbon group; or a and b are 0, R5 represents hydrogen or a substituted or non-substituted hydrocarbon group, and R6 and R7 form a heterocyclic group. In the examples of this patent, a triphenylphosphine oxide as the promoter, which is mainly a phosphorus-containing oxide of the above structure, is disclosed. However, this patent does not disclose the difference in performance between the organic promoters and the aromatic organic additives.
U.S. Pat. No. 4,544,511 discloses a process for producing acetic anhydride by using nickel or a nickel compound as the catalyst together with a metal co-catalyst selected from Groups IA, IIA, IIIB or IVB, and carbon monoxide, methyl acetate or dimethyl ether as the raw materials to carry out a carbonylation reaction at temperatures of 100˜250° C. and CO partial pressures of 3˜150 kg/cm2 in the presence of a halide (bromide or iodide) and at least one trivalent organic nitrogen group promoter. There are three kinds of organic promoters disclosed in this patent:
(I) Compounds of trivalent nitrogen group elements represented by the following formulae:
wherein the formula (2), in which M is N, P, As or Sb, includes triethyl amine, triphenyl amine, N,N-diethyl glycine, etc., and the formula (3) includes N-methyl-2-pyrrodinone, triethylenediamine, etc. when M1 and M2 are N; (II) Hetero cyclic compounds such as picoline, 2,4-lutidine, 2,6-lutidine, 2-hydroxypyridine, 4-picolyamine, 3-pyridinemethanol, picoline-N-oxide, 2-carboxyquinoline, etc.; and (III) Compounds of pentavalent nitrogen group elements.
Although the organic compounds as mentioned in the above patent all need to be used with a metal co-catalyst such as an iodide containing lithium, Tin, aluminum, etc., yet there is no disclosure of the improved performance of the reaction after the addition of organic promoters to the metal iodides that have been contained in the original reaction composition.
EP 0153834 discloses a stabilizer selected from a thiol or an imidazole for preventing the precipitation of the rhodium catalyst in a water-containing carbonylation process. The structure of the imidazole as used in this patent is as below:
in which R1˜R4 are each independently selected from hydrogen, alkyl, aryl, cycloalkyl or alkaryl hydrocarbyl radicals, and the preferred example is N-methylimidazole. However, this patent does not disclose the influence of the catalyst stabilizer on the reaction rate under low water content conditions or even anhydrous conditions, and the examples of applying the catalyst stabilizer in an acetic anhydrides process. Also, the catalyst stabilizer is liable to, with rhodium, form a hardly soluble complex, which will be precipitated from the solution.
EP 0391680 A1 discloses a process for preparing carboxylic acids by using an alcohol or an ester thereof under water-containing conditions and using a quaternary ammonium iodide as a stabilizer of the rhodium catalyst. The structure of the quaternary ammonium iodide is shown as below:
in which R and R1 are independently selected from hydrogen or an alkyl group having 1˜20 carbon atoms and at least one R1 is not hydrogen, and the preferred example is 2-ethyl-4-methylimidazole, 4-methylimidazole, 4-ethylpyridine, 4-t-butylpyridine and 3,4-lutidine. However, although this patent discloses the stabilizing effect of the iodide stabilizer but does not investigate the influence of the acetic anhydride process under anhydrous conditions on the reaction rate.
CN 1876239A and CN 1778468A both disclose a catalytic system for the synthesis of the carbonyl group of methyl acetate to an acid anhydride by using a rhodium compound as the catalyst and different contents of alkyl iodides, hetero-polyacid salts and alkali metal iodine salts as the promoter. The performance of this catalytic system is improved by the synergistic effect of the hetero-polyacid salts, which belong to inorganic compound additives, and the catalyst. However, these patents do not investigate the performance of addition of organic additives.
U.S. Pat. No. 5,298,586 discloses a process for the production of carboxylic acid anhydrides by using an alkyl ester or an alkyl ether as the raw material to carry out the rhodium-catalyzed carbonylation reaction under anhydrous conditions, in which an organic promoter is added to improve the solubility and stability of the rhodium catalysts. The structure of the organic promoter as disclosed in this patent is shown as below:
including 1,3-dialkyl-4-methylimidazolium iodide, 1,3-dialkyl-2,4,5-trimethylimidazoliumiodide, etc., and the preferred promoters are 1,3,4-trimethylimidazolium iodide and 1,2,3,4,5-pentamethylimidazolium iodide. However, this patent does not investigate the influence of addition of organic promoters on the space-time yield of acetic anhydrides.
Therefore, there is still a demand for a process for producing acetic anhydrides under severe carbonylation conditions which can effectively stabilize the rhodium catalyst and maintain a high reaction rate at the same time.